1. Field of the Invention
This invention relates to water treatment systems, and more particularly, to a hydraulic floculation system.
2. Description of the Prior Art
Although in the field of water treatment filtration is commonly thought of as effective in removing fine particles from liquids, such is usually not the case. In practice, effective filtration requires that the liquid be pretreated to cause the particles to group together in "flocs". Also particles settle out in settling basins more rapidly in the form of flocs than small, individual particles. This floc producing process is performed in two steps, namely "coagulation" and "flocculation".
The process of "coagulation" refers to the driving together of colloidal particles by chemical forces. The process occurs within seconds of the application of the coagulation reagents to the liquid, normally water. Because of this property, good mixing is necessary at the point of chemical application in order to insure uniform chemical distribution and exposure of the fine particles in the water to the coagulating agent before the coagulation reaction is completed.
The term "flocculation" refers to the assembling of coagulated particles into floc particles. Flocculation may be partly a chemical bridging mechanism, enhanced by the use of substances like polyelectrolytes, but it is much slower, and more dependent on time and amount of agitation, than coagulation.
There are basically two varieties of flocculation, namely, mechanical flocculation and hydraulic flocculation. Mechanical flocculators generally consist of two types, rotary units and reciprocating units that are usually operated through a crank mechanism. The usual mechanical flocculator drive is an electric motor operating through a gear-type speed reducer. Because the flow rate of water through the mechanical flocculation system may vary, variable speed drive equipment is desirable.
In hydraulic flocculation, power is dissipated in the water by friction generally by flowing the water through baffled tanks. The baffles may be horizontally causing the water to flow horizontally from end to end or vertical causing the water to flow under and over the baffles. Hydraulic flocculation systems have many advantages over mechanical flocculation systems. In addition to operating more uniformly, hydraulic flocculation systems can generally be produced at a lower cost, and they are essentially maintenance free since they do not require any moving parts.
Since flocculation is a mixing process, it is important to restrict particles from flowing from the inlet of a flocculation system directly to the outlet without interacting with other particles. The mixing figure of merit for a flocculation system is the "velocity gradient" or "G-factor". G-factors may be calculated as follows: For hydraulic flocculation: EQU G=.sqroot.62.4 .DELTA.H/.mu.T (Formula 1)
and for mechanical flocculation: EQU G=.sqroot.550P/.mu.V (Formula 2)
in which the G-factor is given in fps per foot, .DELTA.H is the head loss due to friction, in feet, .mu. is viscosity (0.273.times.10.sup.4 pounds-seconds/sf at 50.degree. F. for water), T is the detention time, in seconds, V is volume of the basin, in cubic feet and P is the horsepower dissipated in the water.
An examination of the formula for calculating the G-factor for hydraulic flocculation indicates that the G-factor is constant as long as the head loss .DELTA.H and detention time T are constant.
Flocculating systems often include several flocculating stages. Tapered flocculation is frequently used, with the first flocculated stage operating at a high G-factor, the next stage operating at a lower G-factor and the final stage operating at a still lower G-factor. This accomplishes a maximum input of power, yet reduces particle shearing in the later stages thus building up larger particles that will either settle rapidly in settling tanks or be more efficiently removed by filters.
Excessive G-factors tend to shear floc particles and prevent them from building up to a size that will settle rapidly in settling tanks or be efficiently removed by filters. Insufficient G-factors fail to provide sufficient agitation to enable flocculation to become complete, and may fail to obtain the desired compaction. One problem with mechanical flocculation systems particularly of the rotary variety is that some portions of the mechanical agitator move more rapidly than other portions so that G-factor is not uniform. The outer portion of the agitator will often produce excessive G-factors while the inner portions of the agitator operate at an insufficient G-factor. G-factor non-uniformities can be alleviated to a large extent by hydraulic flocculation. However, the G-factor of a hydraulic flocculation system depends on the rate of flow of water through the system. This does not present a problem where the flow rate of water through the system is constant such as in on-off systems utilizing a storage reservoir. However, hydraulic flocculation systems have been incapable of operating effectively where the flow rate of water substantially fluctuates.
Another disadvantage of hydraulic flocculation systems is the inability to readily vary the G-factor of the system responsive to such variables as changes in the properties of the water or the coagulating chemicals added upstream. With mechanical flocculating systems the G-factor can be varied simply by varying the speed of the drive mechanism. With hydraulic flocculation systems the G-factor is determined entirely by the flow rate thus making it impossible to readily vary the G-factor at which such systems operate.
One variety of hydraulic flocculation system which has been manufactured and sold for many years by Keystone Engineering and Products Company of Seattle, Washington, utilizes a plurality of tanks, each of which receive water either from an external source or another tank through an elbow conduit having an upwardly disposed outlet. The elbows may be mounted in the tank by removable couplings so that they may be quickly interchanged with different sized conduits in order to vary the G-factor of the system. However, even this design is incapable of compensating for variations in flow rate to maintain a constant G-factor.